In the past several decades, the use of optical endoscopes has become common for the visual inspection of inaccessible objects, such as the internal organs of the human body or the internal parts of machinery. These visual inspections are performed in order to assess the need for surgery or equipment tear down and repair; thus the results of the inspections are accorded a great deal of importance. Accordingly, there has been much effort to improve the art in the field of endoscopes.
Endoscopes are long and narrow optical systems, typically circular in cross-section, which can be inserted through a small opening in an enclosure to give a view of the interior. They almost always include a source of illumination which is conducted along the interior of the scope from the outside (proximal) end to the inside (distal) end, so that the interior of a chamber can be viewed even if it contains no illumination. Endoscopes come in two basic types; these are the flexible endoscopes (fiberscopes and videoscopes) and the rigid borescopes. Flexible scopes are more versatile, but borescopes can provide higher image quality, are less expensive, are easier to manipulate, and are thus generally preferred in those applications for which they are suited.
While endoscopes (both flexible and rigid) can give the user a relatively clear view of an inaccessible region, there is no inherent ability for the user to make a quantitative measurement of the size of the objects he or she is viewing. There are many applications for which the size of an object, such as a tumor in a human body, or a crack in a machine part, is a critically important piece of information. Making a truly accurate measurement under these circumstances is a long-standing problem that has not been adequately dealt with until recently.
In my application, Ser. No. 08/689,993, now U.S. Pat. No. 6,009,189 entitled "Apparatus And Method For Making Accurate Three-Dimensional Size Measurements Of Inaccessible Objects", filed Aug. 16, 1996, and which is incorporated herein by reference, I teach a complete system for making measurements of objects viewed through endoscopes. With this system, it is for the first time possible to make measurements which are truly accurate in endoscopic applications. What I mean by "truly accurate" is that the level of accuracy is limited only by the technology of mechanical metrology and by unavoidable errors made by the most careful user.
My system also offers the user the capability to make a usefully accurate measurement at low cost. By "usefully accurate", I mean that the accuracy of the measurement is adequate for the purposes of most common industrial applications. By "low cost", I mean that the user can add this measurement capability to his or her existing remote visual inspection capability with a lower incremental expenditure than is required with previously available systems.
I call this new method "perspective dimensional measurement". By "perspective" I am referring to the use of two or more views of an object, obtained from different viewing positions, for dimensional measurement of the object. By "dimensional measurement", I mean the determination of the true three-dimensional (height, width, and depth) distance between two or more selected points on the object.
To perform a perspective dimensional measurement, the apparent positions of each of the selected points on the object are determined in each of the views. This is the same principle used in stereoscopic viewing, but here one is concerned with making quantitative measurements of object dimensions, rather than obtaining a view of the object containing qualitative depth cues. As I taught in the referenced patent application, given sufficient knowledge about the relative locations, orientations and imaging properties of the viewing optical system(s) or camera(s), one can determine the locations of the selected points in a measurement coordinate system. Once these locations are known, one then simply calculates the desired distances between the selected points by use of the well-known Pythagorean Theorem.
As a necessary and integral part of my complete measurement system, I taught how to calibrate it in the referenced co-pending application. I taught the use of a complete set of robust calibration procedures, which removes the need for the measurement system to be built accurately to a specific geometry, and also removes any need for the camera(s) to be built accurately to specific optical characteristics. Instead, I taught how to calibrate the geometry of the opto-mechanical hardware, and how to take that actual geometry into account in the measurement process. The complete set of calibration procedures I taught includes three different types of calibration. In optical calibration, the characteristics of each camera, when used as a precision image forming device, are correctly determined. In alignment calibration, the orientation of each camera's measurement coordinate axes with respect to the translation of the camera are determined. Finally, in motion calibration, any errors in the actual motion of the camera(s), as compared to the ideal motion, are determined.
In certain embodiments, my system of perspective dimensional measurement, as taught in the referenced co-pending application, enables one to make accurate measurements using a standard, substantially side-looking, rigid borescope. Since the person who needs the measurement will often already own such a borescope, the new method has a significant cost advantage over previous measurement techniques.
In a preferred set of embodiments, the motion of the borescope is constrained to lie along a substantially straight line. The borescope is supported by and its position is controlled by a mechanical assembly that I call the borescope positioning assembly (BPA). The borescope is attached to the BPA with a clamp which allows the borescope to slide and rotate with respect to the BPA so that the user may conveniently acquire the object of interest in the borescope's field of view. Once the object is acquired, the clamp is tightened to securely attach the borescope to the BPA, and then the measurement can proceed.
While these embodiments produce accurate dimensional measurements using a standard borescope, there is a difficulty. The problem is that a new alignment calibration may have to be performed each time a new measurement situation is set up. In alignment calibration, the orientation of the borescope's measurement coordinate axes with respect to the motion provided by the BPA is determined. With a standard borescope, this orientation may not be well controlled, and thus every time the borescope is repositioned with respect to the BPA, there is the logical requirement for a new alignment calibration. Of course, whether a new calibration would actually be required in any specific instance depends on the accuracy required of the dimensional measurement in that instance.